Multiple-input multiple-output (MIMO) transmission refers to the use of multiple antennas both on the transmitter side and on the receiver side. Beamforming is an example of a MIMO technique. In beamforming the same signal is emitted from each of the transmit antennas with appropriate phase weighting such that the signal power is maximized at the receiver output. Sometimes gain weighting is also applied to the signals of each of the transmit antennas.
Space time coding is another example of a MIMO technique. Space time coding is a technique to transmit multiple copies of a data stream across a number of antennas and to exploit the various received versions of the data to improve the reliability of data transfer. For space time coding there exists for example the Alamouti scheme which is originally designed for two transmit antennas. In diversity coding techniques like space time coding a single stream is transmitted in a coded way. The signal is emitted from each of the transmit antennas using certain principles of full or near orthogonal coding. In for example OFDM (orthogonal frequency division multiplex) systems an open loop transmit diversity technique, as e.g. the Alamouti scheme, can be used as space time coding or space frequency coding.
Another known MIMO technique is spatial multiplexing. In spatial multiplexing a data stream is split into multiple streams and each stream is transmitted from a different transmit antenna in the same frequency channel. Therefore the space dimension is re-used or multiplexed more than once. Transmit diversity and spatial multiplexing with per antenna rate control (PARC) is another example of a MIMO technique.